Same Note, Different Voice: What Timbre Is
Suppose a flute and a trumpet both play the same note at the same loudness. You would still never mix them up. Something about each sound is different, and that quality is called timbre, which is said like TAM-ber. It is the reason a violin sounds like a violin and not like a piano.
To understand timbre, think about what happens when a string or a column of air vibrates. It does not only move in one big swing. It also vibrates in smaller parts at the same time, and these make quieter, higher tones called overtones. Your ear blends the main note and the overtones into one sound, so you hear a single note.
Different instruments make different mixes of overtones. The shape of an instrument, the material it is made from, and the way it is played all change which overtones are strong and which are weak. A mellow sound may have only a few strong overtones, while a bright, buzzing sound has many.
Timbre helps composers choose instruments for a piece. A sad tune might be given to a warm, dark cello, while a lively one may sparkle on a piccolo. Even people have their own timbre, which is why you can recognize a friend's voice on the phone without seeing a face.